Acknowledge the fact that he's probably not your best friend
We hate to be the bearer of bad (also, obvious) news, but if your boy is raiding your past arsenal of ass, he's probably not your best friend. If he was a decent, bro-code abiding man, he would have observed a proper waiting period (depending on the situation, the proper waiting period could reasonably be never) and asked for your permission. This is the inalienable, impenetrable, fucking basic man code.
Sure, you can't fault him for catching feelings, but you can fault him for acting on them with no regard to yours. He had a choice to put your feelings first, but instead he chose to smash. A real pal prioritizes his friendship over a female, no exceptions.